Electron Cloud Issues for the 3.2km Positron Damping Ring Mark Palmer (Cornell University) GDE January 18, 2011 ILC Baseline Assessment Workshop 2 SLAC

EC Working Group Tasks & Status Jan 18, 2010 ILC BAW-2 Global Design Effort 2 Reduced DR Circumference – ILC Low Power Scenario Evaluation of reducing the DR circumference to one-half that specified for the RDR Corresponding reduction in bunch count to one-half of the RDR specification Baseline Mitigation Recommendation Evaluation of EC Mitigation R&D Results Identify the most promising mitigation schemes Identify candidates and issues for further R&D Restored Bunch Count Evaluation of options to restore the RDR bunch count for high luminosity operation Identify safe path to restore bunch count ✓ Evaluate performance limits in low power configuration March 2010 October 2010 Under Evaluation Participating Institutions: ANL, Cornell, INFN, KEK, LBNL, SLAC  

Circumference Evaluation I Jan 18, 2010 ILC BAW-2 Global Design Effort 3 Circumference change with reduced bunch count maintains beam current and bunch spacing  expect minimal changes in EC instability thresholds

Comparison of 6.4 and 3.2 km DR Options Jan 18, 2010 ILC BAW-2 Global Design Effort 4 antechamber SEY 1.2 SEY 1.4 antechamber Summer 2010 Evaluation Comparison of Single Bunch EC Instability Thresholds for: - 6.4km ring with 2600 bunches - 3.2km ring with 1300 bunches  same average current Both ring configurations exhibit similar performance  3.2km ring (low current option) is an acceptable baseline design choice S. Guiducci, M. Palmer, M. Pivi, J. Urakawa on behalf of the ILC DR Electron Cloud Working Group

C ESR TA Mitigation Tests Jan 18, 2010 ILC BAW-2 Global Design Effort 5 DriftQuadDipoleWigglerVC Fab Al CU, SLAC Cu CU, KEK, LBNL, SLAC TiN on Al CU, SLAC TiN on Cu CU, KEK, LBNL, SLAC Amorphous C on Al CERN, CU Diamond-like C on Al1/2011 CU, KEK NEG on SS CU New NEG FormulationsPhase II ? Cockroft, CU Solenoid Windings CU Fins w/TiN on Al SLAC Triangular Grooves on Cu CU, KEK, LBNL, SLAC Triangular Grooves w/TiN on AlPhase II ? CU, SLAC Triangular Grooves w/TiN on Cu1/2011 CU, KEK, LBNL, SLAC Clearing ElectrodePhase II ? CU, KEK, LBNL, SLAC

EC Mitigation Options I (Drift Studies) Jan 18, 2010 ILC BAW-2 Global Design Effort 6 Solenoid windings PEP II Amorphous Carbon CERN KEK

EC Mitigation Options II Jan 18, 2010 ILC BAW-2 Global Design Effort 7 Clearing Electrodes KEKB Grooves w/TiN coating 7 Clearing Electrode C ESR TA Grooves on CuAfter extended operation Stable Structures Reliable Feedthroughs Manufacturing Techniques & Quality

EC Mitigation Evaluation – 4 Criteria Jan 18, 2010 ILC BAW-2 Global Design Effort 8 Efficacy Photoelectric yield (PEY) Secondary emission yield (SEY) Ability to keep the vertical emittance growth below 10% Cost Design and manufacturing of mitigation Maintenance of mitigation –Ex: Replacement of clearing electrode PS Operational –Ex: Time incurred for replacement of damaged clearing electrode PS Risk Mitigation manufacturing challenges: –Ex: ≤1mm or less in small aperture VC –Ex: Clearing electrode in limited space or in presence of BPM buttons Technical uncertainty –Incomplete evidence of efficacy –Incomplete experimental studies –Ex: No long-term durability study for a-C in synchrotron radiation environment Reliability –Durability of mitigation –Ex: Damage of clearing electrode feed- through –Ex: Failure of clearing electrode PS Impact on Machine Performance Impact on vacuum performance –Ex: NEG pumping can have a positive effect –Ex: Grooves  added surface for pumping –Ex: Vacuum outgassing Impact on machine impedance –Ex: Impedance of grooves and electrodes Impact on optics –Ex: x-y coupling due to solenoids Operational –Ex: NEG re-activation after saturation –Ex: Mitigation availability –Ex: Replacement time for damaged components

Structured Evaluation of EC Mitigations Jan 18, 2010 ILC BAW-2 Global Design Effort 9

Evaluation Mitigations rated on a scale from -4 (poor) to 4 (good) for each criteria and overall rankings evaluated Working group plus additional EC experts carried out evaluations at the conclusion of ECLOUD10 on Oct. 13, Final evaluation included detailed discussion to confirm recommendation in addition to the numerical rankings Need to pursue an aggressive mitigation plan causes a heavy weighting on efficacy –Baseline targets  max ≤1.2 –Move to higher beam current will require even better performance (will return to this later) Jan 18, 2010 ILC BAW-2 Global Design Effort 10

Drift Region Evaluation Jan 18, 2010 ILC BAW-2 Global Design Effort 11  TiN is the recommended baseline mitigation Good efficacy Risks for its implementation are the lowest No significant impact on other aspects of machine performance  Solenoids recommended as an additional mitigation Maximize efficacy – particularly important for higher current operation  NEG coating is recommended as the alternate mitigation Somewhat lower mitigation efficacy Advantage of providing vacuum pumping in the long straights

Dipole Region Evaluation Jan 18, 2010 ILC BAW-2 Global Design Effort 12  Grooves with TiN coating are recommended as the baseline mitigation Very good efficacy  Antechambers recommended as additional mitigation Important for photoelectron control  TiN coating without grooves recommended as alternate Clearing electrodes offer best efficacy but have risks and machine impact Further R&D could result in an updated choice in this critical region

Wiggler Region Evaluation Jan 18, 2010 ILC BAW-2 Global Design Effort 13  Clearing Electrodes deposited via thermal spray on Cu chambers are the recommended baseline mitigation Best efficacy (required in this region) Risks and impact are limited due to having fewer affected chambers  Antechambers recommended as additional mitigation Important for photoelectron control and for power removal  Grooves with TiN coating recommended as the alternate mitigation scheme

Quadrupole Region Evaluation Jan 18, 2010 ILC BAW-2 Global Design Effort 14  TiN is the recommended baseline mitigation Good efficacy Risks for its implementation are the lowest No significant impact on other aspects of machine performance Concerns about long-term build-up in the quadrupoles Requires particularly effective EC suppression  either grooves or electrodes Further R&D required to validate either option

Preliminary C ESR TA results and simulations suggest the possible presence of sub-threshold emittance growth - Further investigation required  May require reduction in acceptable cloud density  reduction in safety margin An aggressive mitigation plan is required to obtain optimum performance from the 3.2km positron damping ring and to pursue the high current option *Drift and Quadrupole chambers in arc and wiggler regions will incorporate antechambers Summary of Mitigation Plan Jan 18, 2010 ILC BAW-2 Global Design Effort 15 Mitigation Evaluation conducted at satellite meeting of ECLOUD`10 (October 13, 2010, Cornell University) S. Guiducci, M. Palmer, M. Pivi, J. Urakawa on behalf of the ILC DR Electron Cloud Working Group

Further Comments A concern for meeting the emittance specifications is a steady incoherent emittance growth at low electron cloud densities below the threshold for the head-tail instability. Recent simulations and CesrTA measurements suggest that this effect may be significant and are leading to a re-evaluation of the acceptable electron densities. While considerable work remains to precisely quantify this issue, initial results suggest that the acceptable cloud densities may need to be lowered by a factor of several. This further emphasizes the need to employ the most effective mitigation techniques, consistent with risk and cost constraints, possible in each region of the ring. However, the present mitigation scheme is consistent with the performance requirements of the 3.2km ring design Jan 18, 2010 ILC BAW-2 Global Design Effort 16

Restoration of Bunch Count Fall-back plan (safe option) is to add a second positron damping ring However, the EC working group is actively studying the possibility of 3ns bunch spacing in a single positron damping ring –Achieving this goal requires pursuit of the most efficacious mitigation scheme –A look at the dipole region on the following slide… Jan 18, 2010 ILC BAW-2 Global Design Effort 17

Dipole Region Evaluation (DSB3) for Possible High Current Operation Jan 18, 2010 ILC BAW-2 Global Design Effort 18 6 ns, w/o antech. 6 ns, w. antech. 3 ns, w. antech. 3 ns, w/o antech. Space-averaged EC Density Present Target To achieve equivalent EC Density

Summary Jan 18, 2010 ILC BAW-2 Global Design Effort 19 Preliminary recommendations for the EC mitigation scheme are in place The proposed plan is consistent with the choice of a 3.2km ring in the low power option The acceptable EC threshold to prevent emittance growth may depend significantly on issues that are still under investigation –Possibility of incoherent emittance growth below the head-tail instability threshold (continued experimental and simulation effort) –The effective photoelectron production rate (improved simulations being prepared)  Warrants an aggressive EC mitigation plan The possibility of restoring the high luminosity bunch count without resorting to an additional positron ring requires further evaluation and the most aggressive approach to the mitigation scheme. Good News – we expect that an equivalent mitigation scheme will be tested in SuperKEKB prior to ILC construction